1. Field of the Invention
The present invention relates to a method of unbalanced disc detection, and particularly to a method of unbalanced disc detection in a slim type optical drive.
2. Description of the Related Art
Generally, an optical disc is designed to have a symmetrical shape, with its geometric and gravitational axis positioned on the center of the disc. However, in practical use, the disc may be not ideally symmetrical in shape or in weight due to material distribution defects or poor manufacturing. In this case, either the geometric and/or gravitational axes deviate from the center of the disc, producing a gravitationally and/or geometrically eccentric disc. If such a disc is disposed in the optical drive, imbalance occurs in the disc rotation due to the deviation of geometry or gravity, which generally leads to data retrieval failure. This will be further described in the following paragraph.
When the above-mentioned imbalance occurs in the disc rotation, a force due to this imbalance is generated and applied to the disc, which further leads to disc wobble and annoying noise in disc rotation. The disc wobble and the noise not only disturb the user, but also cause potential damage to the optical drive structure. If the optical drive is a slim type optical drive, which has a relatively small structure and is generally applied in notebooks or handhelds, the effect of the force due to this imbalance may be further aggravated.
The above-mentioned force due to the imbalance can be divided into a vertical force (i.e. direction of the center of the disc) and a horizontal force (i.e. direction parallel to the disc surface). Disc wobble is mainly caused by the vertical force, and axial force F between the rotational driving spindle and the holding mechanism of the optical drive and the disc is mainly caused by the horizontal force. In this case, if a disc with a weight M is applied, a relation between the axial force F and the weight M of the disc is shown as Equation (1):Fm=M×r×ω2  (1)
In Equation (1), Fm refers to a component of the axial force F due to the weight M of the disc, r refers to a distance from the gravitational axis of the disc to the center of the disc, and ω refers to the rotation speed of the disc.
From Equation (1), the axial force Fm due to the weight of the disc is in direct proportion to the distance r from the gravitational axis of the disc to the center of the disc and to the square of the rotation speed ω of the disc.
When the disc is a geometrically eccentric disc whose gravitational axis does not deviate from the center of the disc, the distance r from the gravitational axis of the disc to the center of the disc equals zero. Thus, Fm equals zero, and the axial force F is fixed even if variation occurs in the rotation speed of the disc. That is, the imbalance of the eccentric disc is fixed even if the rotation speed of the eccentric disc is variable. In this case, the disc wobble and the annoying noise can be alleviated by applying a vibration eliminating device or a noise-eliminating device to the optical drive.
On the other hand, when the disc is gravitationally eccentric, the gravitational axis deviates from the center of the disc by a distance r, which is not zero. That is, Fm is in direct proportion to ω2. Consequently, when the rotation speed of the unbalanced disc is increased, the imbalance of the unbalanced disc is rapidly increased. Thus, the imbalance of the unbalanced disc not only exists in the disc rotation, but is also amplified when the rotation speed of the disc is increased. In this case, even if the above-mentioned vibration eliminating device or noise-eliminating device are applied to the optical drive, the disc wobble and the annoying noise may surpass the limitation of these devices due to the increasing rotation speed.
Consequently, when a gravitationally eccentric disc is applied to the optical drive, data retrieval of the unbalanced disc is performed at a relatively low rotation speed to avoid rapidly increasing imbalance. In this case, a method of unbalanced disc detection must be applied. If the disc is determined to be gravitationally eccentric, the rotation speed of the rotational driving spindle is lowered to alleviate the disc wobble and the annoying noise caused by the imbalance.
Generally, a servo control system of the optical drive, i.e. the slim type optical drive, is used to perform disc detection. An example of the structure of the conventional servo control system of the slim type optical drive is described in detail with reference to FIG. 1.
The servo control system of the optical drive performs focus and tracking control. In FIG. 1, the optical pickup 10 of the optical drive performs data retrieval, and the position signal of the optical pickup 10 is transformed by the front-end processor (FEP) 20 to an analog focusing error signal FE and an analog tracking error signal TE. The analog focusing error signal FE and the analog tracking error signal TE are then transmitted to the digital signal processor (DSP) 30 to digitize the signals to a digital focusing error signal DFE and an off-track signal OFTR. The digital focusing error signal DFE and the off-track signal OFTR are then transmitted to the micro control unit (MCU) 40 for further action, such as focus or tracking control of the optical drive to the optical pickup 10.
The conventional method of unbalanced disc detection applied to the above-mentioned servo control system mainly applies a manner of detecting position deviation of the optical pickup, which corresponds to an incline angle of the optical pickup to the disc. In this case, when disc wobble occurs, the incline angle is variable, and deviation can be determined by the variation in the position deviation of the optical pickup. Thus, the imbalance of the unbalanced disc can be quantified in the form of the variation in the position deviation of the optical pickup.
However, the conventional servo control system of the optical drive does not provide any function that directly obtains the incline angle or the position deviation of the optical pickup. Consequently, either additional electronic components or a specific FEP 20 with additional function are required on the main board of the optical drive to detect the incline angle or the position deviation of the optical pickup. Thus, not only is the cost of the optical drive increased, but the main board of the optical drive m be enlarged, which definitely leads to disadvantage in design and manufacture of the optical drive.